>> ZRM is within a couple of thousandths of the base of the cap. In this >> case it's in the root. > > > Interesting. I wonder why? I wonder if it has something to do with the > different grain angles of the bridge body and cap, so that when you run > the drill bit down through both, the pin ends up being tighter in the > cap than the body. I'm not sure. I assumed the glue line the pin passed through was a factor. I'll have to make up a horizontally laminated root and see what happens. >> I doubt it, but the cap could be made much denser and more >> dimensionally stable. Just going to a laminated cap (1.5mm >> laminations) lowered the 0.011" pin height difference to 0.004". An >> even denser cap should be even better. > > > Do you think there's a correlation between density and stability? Not per se, but I expect these thin laminations saturated with epoxy won't be soaking up much moisture, and will be pretty difficult to crush. > It sounds like you're making Permali. > > http://www.permalidehoplast.co.uk/ It does, or a considerably less sophisticated version, at any rate. > I think somewhere I have a sample of the Hydulignum made by this > company. Maybe I could whack a piece off for you to experiment with, > assuming I have anything that will cut it (which isn't a trivial > assumption - I think I got this sample when visiting a waterjet cutting > operation - I think the point of it was something like, we can even cut > this stuff). Spitfire propeller blades! I'd love a sample if the hacksaw survives. > I wouldn't think that maple would absorb the epoxy well since it's not > porous. This is 0.6mm sliced veneer. By the time a blade is pushed through a saturated flitch to shave it off, there's not much structural integrity left cross grain. The epoxy presses clear through the stuff in the clamps. > > If you're essentially making a cap out of epoxy, why not make it a > carbon fiber composite, rather than a wood fiber composite? It seems > that it would be stronger and harder. Functionally, there's no reason not to other than notching difficulty (and maybe carbon fiber dust). Psychologically, might be another problem. The appearance of normalcy carries more weight than it probably should. Eventually... > While we're on > that subject, I don't know how much of the resistance to rendering comes > from the string friction on the bridge cap. I think the resistance of > the cap to down bearing (and to humidity movement) takes place mostly > near the bridge pins. So the area of the cap between the pins is really > doing nothing but increasing the resistance to rendering. Perhaps the > cap should have a relief between the pins. Given the same friction coefficient between the string and cap for all forces, the down vector force alone, 5.4lb for each pin, sort of overpowers the 2.7lb from downbearing - and it's at the pin, not in the middle. Where does the curved bridge top to take downbearing force off of the notch edge fit in here? > OK. But the crushing seems to be more severe at the notch edge. Because > of the path that the string is taking it has a fulcrum point, if you > will, at the notch edge (which I believe Ric mentioned). So, the bridge > is trying to lift the string up at the edges of the two notches and > they're being crushed. Look at your own PSI figures and decide how much of the damage is done by downbearing and how much by pin friction and bridge dimensional changes. Then look at the photo I posted, and tell me how a string on that bridge crushed the notch edge at that angle from downbearing. I don't believe that piano ever had a 20° front bearing angle. In the world of science, how could downbearing alone EVER crush a notch edge past the point where the string can touch it????? I don't see this as being geometrically possible. > I'm not sure that I agree. Because of the present geometry, as the > bridge moves up, it is essentially lifting the string up at two points - > the notch edges. If the top of the bridge described the curve that the > string naturally wanted to take, as the bridge tried to lift the string > up, it would be lifting it along the entire length of contact with the > bridge, which would lower the bearing stress considerably and perhaps > prevent indenting of the cap. > > Phil F The friction is still there, and the PSI load is still there. The cap would still crush at the edges as the bridge cap expanded, and the string would lose contact with the notch edge in dry seasons even sooner than with a flat cap. Ron N
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